[build] main *conceptually* targets .NET 7 (#978)

Fixes: #962

Context: 3851b1a
Context: #936
Context: #975

We have several PRs which impact public API, and we want to land as
part of .NET 7, and *not* as part of .NET 6, e.g. #936.

Update `GitInfo.txt` to `7.0`, and update `$(_NetCoreLibVersion)` to
be `$(GitBaseVersionMajor).$(GitBaseVersionMinor).0.0` -- reverting
commit 3851b1a -- as an indicator that from this point onward, all
work on `main` is targeting .NET 7.

Note that CI continues to use .NET 6.  This is thus a more
"conceptual" change rather than a "everything requires .NET 7 now"
change.

Author: jonpryor

GitInfo.txt

@@ -1 +1 @@
-6.0
+7.0

build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.targets

@@ -113,7 +113,7 @@
     <PropertyGroup>
       <_NetToolVersion>$(GitBaseVersionMajor).$(GitBaseVersionMinor).$(GitBaseVersionPatch).$(GitCommits)</_NetToolVersion>
       <_OldToolVersion>0.2.$(GitBaseVersionPatch).$(GitCommits)</_OldToolVersion>
-      <_NetCoreLibVersion>0.1.0.0</_NetCoreLibVersion>
+      <_NetCoreLibVersion>$(GitBaseVersionMajor).$(GitBaseVersionMinor).0.0</_NetCoreLibVersion>
       <_OldCoreLibVersion>0.1.0.0</_OldCoreLibVersion>
       <_FileVersion>$(GitBaseVersionMajor).$(GitBaseVersionMinor).$(GitBaseVersionPatch).$(GitCommits)</_FileVersion>
     </PropertyGroup>